Fluid-mediated, brittle–ductile deformation at seismogenic depth – Part 2: Stress history and fluid pressure variations in a shear zone in a nuclear waste repository (Olkiluoto Island, Finland)
<p>The microstructural record of fault rocks active at the brittle–ductile transition zone (BDTZ) may retain information on the rheological parameters driving the switch in deformation mode and on the role of stress and fluid pressure in controlling different fault slip behaviours. In this stu...
| Main Authors: | , , , , , |
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| Format: | Article |
| Language: | English |
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Copernicus Publications
2020-04-01
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| Series: | Solid Earth |
| Online Access: | https://www.solid-earth.net/11/489/2020/se-11-489-2020.pdf |
| _version_ | 1818281899621089280 |
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| author | F. Prando L. Menegon L. Menegon M. Anderson B. Marchesini J. Mattila J. Mattila G. Viola |
| author_facet | F. Prando L. Menegon L. Menegon M. Anderson B. Marchesini J. Mattila J. Mattila G. Viola |
| author_sort | F. Prando |
| collection | DOAJ |
| description | <p>The microstructural record of fault rocks active at the brittle–ductile transition zone (BDTZ) may retain information on the rheological parameters driving the switch in deformation mode and on the role of stress and fluid pressure in controlling different fault slip behaviours. In this study we analysed the deformation microstructures of the strike-slip fault zone BFZ045 in Olkiluoto (SW Finland), located in the site of a deep geological repository for nuclear waste. We combined microstructural analysis, electron backscatter diffraction (EBSD), and mineral chemistry data to reconstruct the variations in pressure, temperature, fluid pressure, and differential stress that mediated deformation and strain localization along BFZ045 across the BDTZ. BFZ045 exhibits a mixed ductile–brittle deformation, with a narrow (<span class="inline-formula"><20</span> cm thick) brittle fault core with cataclasites and pseudotachylytes that overprint a wider (60–100 cm thick) quartz-rich mylonite. Mylonitic deformation took place at 400–500 <span class="inline-formula"><sup>∘</sup></span>C and 3–4 kbar, typical of the greenschist facies metamorphism at the base of the seismogenic crust. We used the recrystallized grain size piezometry for quartz to document a progressive increase in differential stress, from ca. 50 to ca. 120 MPa, towards the shear zone centre during mylonitization and strain localization. Syn-kinematic quartz veins formed along the mylonitic foliation due to transiently high pore fluid pressure (up to lithostatic value). The overprint of the veins by dynamic recrystallization and mylonitic creep is further evidence of the occurrence of brittle events under overall ductile conditions. We propose a conceptual model in which the ductile–brittle deformation cycle was controlled by transient oscillations in fluid pressure and progressively higher differential stress, possibly occurring in a narrowing shear zone deforming towards the peak strength of the crust at the BDTZ.</p> |
| first_indexed | 2024-12-13T00:12:27Z |
| format | Article |
| id | doaj.art-c1515efb2aaa41868aa53aba447fb6b9 |
| institution | Directory Open Access Journal |
| issn | 1869-9510 1869-9529 |
| language | English |
| last_indexed | 2024-12-13T00:12:27Z |
| publishDate | 2020-04-01 |
| publisher | Copernicus Publications |
| record_format | Article |
| series | Solid Earth |
| spelling | doaj.art-c1515efb2aaa41868aa53aba447fb6b92022-12-22T00:05:55ZengCopernicus PublicationsSolid Earth1869-95101869-95292020-04-011148951110.5194/se-11-489-2020Fluid-mediated, brittle–ductile deformation at seismogenic depth – Part 2: Stress history and fluid pressure variations in a shear zone in a nuclear waste repository (Olkiluoto Island, Finland)F. Prando0L. Menegon1L. Menegon2M. Anderson3B. Marchesini4J. Mattila5J. Mattila6G. Viola7School of Geography, Earth and Environmental Sciences, University of Plymouth, PL48AA Plymouth, UKSchool of Geography, Earth and Environmental Sciences, University of Plymouth, PL48AA Plymouth, UKThe Njord Centre, Department of Geoscience, University of Oslo, P.O. Box 1048 Blindern, NorwaySchool of Geography, Earth and Environmental Sciences, University of Plymouth, PL48AA Plymouth, UKDipartimento di Scienze Biologiche, Geologiche e Ambientali, Università di Bologna, ItalyGeological Survey of Finland, Espoo, Finland currently at: Rock Mechanics Consulting Finland Oy (RMCF), Vantaa, FinlandDipartimento di Scienze Biologiche, Geologiche e Ambientali, Università di Bologna, Italy<p>The microstructural record of fault rocks active at the brittle–ductile transition zone (BDTZ) may retain information on the rheological parameters driving the switch in deformation mode and on the role of stress and fluid pressure in controlling different fault slip behaviours. In this study we analysed the deformation microstructures of the strike-slip fault zone BFZ045 in Olkiluoto (SW Finland), located in the site of a deep geological repository for nuclear waste. We combined microstructural analysis, electron backscatter diffraction (EBSD), and mineral chemistry data to reconstruct the variations in pressure, temperature, fluid pressure, and differential stress that mediated deformation and strain localization along BFZ045 across the BDTZ. BFZ045 exhibits a mixed ductile–brittle deformation, with a narrow (<span class="inline-formula"><20</span> cm thick) brittle fault core with cataclasites and pseudotachylytes that overprint a wider (60–100 cm thick) quartz-rich mylonite. Mylonitic deformation took place at 400–500 <span class="inline-formula"><sup>∘</sup></span>C and 3–4 kbar, typical of the greenschist facies metamorphism at the base of the seismogenic crust. We used the recrystallized grain size piezometry for quartz to document a progressive increase in differential stress, from ca. 50 to ca. 120 MPa, towards the shear zone centre during mylonitization and strain localization. Syn-kinematic quartz veins formed along the mylonitic foliation due to transiently high pore fluid pressure (up to lithostatic value). The overprint of the veins by dynamic recrystallization and mylonitic creep is further evidence of the occurrence of brittle events under overall ductile conditions. We propose a conceptual model in which the ductile–brittle deformation cycle was controlled by transient oscillations in fluid pressure and progressively higher differential stress, possibly occurring in a narrowing shear zone deforming towards the peak strength of the crust at the BDTZ.</p>https://www.solid-earth.net/11/489/2020/se-11-489-2020.pdf |
| spellingShingle | F. Prando L. Menegon L. Menegon M. Anderson B. Marchesini J. Mattila J. Mattila G. Viola Fluid-mediated, brittle–ductile deformation at seismogenic depth – Part 2: Stress history and fluid pressure variations in a shear zone in a nuclear waste repository (Olkiluoto Island, Finland) Solid Earth |
| title | Fluid-mediated, brittle–ductile deformation at seismogenic depth – Part 2: Stress history and fluid pressure variations in a shear zone in a nuclear waste repository (Olkiluoto Island, Finland) |
| title_full | Fluid-mediated, brittle–ductile deformation at seismogenic depth – Part 2: Stress history and fluid pressure variations in a shear zone in a nuclear waste repository (Olkiluoto Island, Finland) |
| title_fullStr | Fluid-mediated, brittle–ductile deformation at seismogenic depth – Part 2: Stress history and fluid pressure variations in a shear zone in a nuclear waste repository (Olkiluoto Island, Finland) |
| title_full_unstemmed | Fluid-mediated, brittle–ductile deformation at seismogenic depth – Part 2: Stress history and fluid pressure variations in a shear zone in a nuclear waste repository (Olkiluoto Island, Finland) |
| title_short | Fluid-mediated, brittle–ductile deformation at seismogenic depth – Part 2: Stress history and fluid pressure variations in a shear zone in a nuclear waste repository (Olkiluoto Island, Finland) |
| title_sort | fluid mediated brittle ductile deformation at seismogenic depth part 2 stress history and fluid pressure variations in a shear zone in a nuclear waste repository olkiluoto island finland |
| url | https://www.solid-earth.net/11/489/2020/se-11-489-2020.pdf |
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